It is known that a radio wave detector used for a 45 mm radio telescope is placed within a vacuum chamber which is cooled to a temperature of about 20 K.
Patent Literature 1 discloses a configuration having a cooling window material which has an enlarged diameter of at least 170 mm together with characteristics of efficiently absorbing thermal infrared rays, improving thermal conductivity, improving transparency in a sub millimeter area and reducing a reflection loss, an extremely low temperature cooling apparatus for an astronomical observation device including the window material, and examination equipment utilizing terahertz wave observation. As for the window material, Patent Literature 1 discloses a configuration in which an electrical wave introduction window 13 of a cooling container 11 includes a cooling window material 14 in which a monocrystal sapphire substrate is used as a base material, a cut face is formed as a C plane, and a thick anti-reflection film of silicon oxide (SiOx) is formed on a surface of the sapphire substrate. However, this methodology is effective for external thermal insulation, but provides no description and no suggestion regarding means for exhausting heat radiated in the cooling container from electromagnetic waves passing through the cooling window material.
Patent Literature 2 discloses a system in which a body to be tested is placed within a vacuum chamber of a Dewar flask structure and the vacuum chamber is cooled to an extremely low temperature. However, this methodology is effective for the external thermal insulation, but provides no description and no suggestion regarding means for exhausting heat radiated in the vacuum chamber.
Patent Literature 3 discloses a cryogenic container in which nested vacuum chambers are provided and each chamber has a multi-layer radiation shield plate on an inner wall surface thereof, and the innermost chamber is filled with a refrigerant within which a superconductive winding is placed. However, this methodology is effective for the external thermal insulation, but provides no description and no suggestion regarding means for exhausting heat radiated in the cooling container.
Generally, a filter is practically provided to a window serving to take in radio waves for astronomical observation. Examples of the filter used include an optical glass filter or plastic filter such as an ultraviolet filter, a visible light filter, an infrared filter and the like.
Absorption heat of the filter is generally exhausted by a refrigerating machine and the like. If a refrigerating machine is used in a radio wave detector having a large aperture, cooling of the filter disadvantageously loads on the refrigerating machine. Since the optical glass filter and the plastic filter have a large refractive index and reflect a part of the radio waves in nature, the thicker the filter is, the less the radio waves are transmitted therethrough. Therefore, electromagnetic waves such as ultraviolet rays, visible light rays, infrared rays, far infrared rays and the like are transmitted through the filter of the window to some degree. The optical glass filter and the plastic filter need to be subjected to antireflective treatment, and the antireflective treatment requires high technology.
Sensitivity of a radio wave measuring instrument improves in proportion to an opening area of a window for radio wave incident, but then again, window aperture enlargement is very difficult because the window aperture enlargement involves increase in heat intrusion.
In order to enhance radio wave detection sensitivity, it is very effective to enlarge the aperture of the window of the vacuum vessel through which the radio waves come in and go out. Moreover, it may be considered to be desirable that, in order to exhaust the heat in the vacuum vessel, a material having high radio wave transparent characteristics and high thermal insulation characteristics be provided within the vacuum vessel, but there are almost no reports of such devices.
As described above, in the radio wave measurement device having the radio wave detector placed within a vacuum low temperature vessel, the low temperature vessel is kept at a low temperature by a thermal insulation material, but the thermal insulation principle is similar to the principle of a vacuum multi-layer insulation (MLI) used in a general refrigerating machine, that is, a thermal insulation principle using radiative equilibrium, and therefore, the MLI is provided along the wall of the low temperature vessel so as to surround the low temperature vessel. However, this methodology has a problem in that the extremely low temperature is hard achieved since the heat radiated in the vacuum low temperature vessel is not sufficiently exhausted. However, an effective means for solving this problem has not been reported in the past.
The radio wave measurement device is used as, for example, a radio wave measurement device for a radiometer. A radiometer is disclosed in, for example, Patent Literature 4.
Patent Literature 4 discloses that radio wave detection is performed at room temperature, but provides no description and no suggestion regarding placing radio wave detector at a low temperature. Patent Literature 4 discloses only cooling means of immersing a black body in liquid nitrogen for calibrating a temperature.